A typical one of the molecular sieves having MFI structure is ZSM-5 zeolite that was developed by US Mobil Corp. in early 1970s. It has unique porous channel structure and has been used broadly in reaction processes such as alkylation, isomerization, disproportion, catalytic cracking, catalytic dewaxing and the like. Catalytic cracking is one of important process techniques for producing light olefins and improving gasoline octane number. For most of catalytic cracking units, using a catalyst or additive that contains a molecular sieve having MFI structure is an effective way for increasing the production of propylene and butylene, and the octane number of gasoline.
In U.S. Pat. No. 3,758,403, a process is disclosed for boosting the octane number of gasoline and increasing the yield of C3-C4 olefins by adding ZSM-5 molecular sieve into the catalytic cracking catalyst. For example, when a conventional catalyst containing 10% REY is added with ZSM-5 molecular sieve from 1.5%, 2.5%, 5% or up to 10%, the gasoline octane number and the yield of lower olefins are increased. However, the increasing amplitude is reduced with increasing the amount of ZSM-5 molecular sieve added. Using an additive that contains ZSM-5 molecular sieve has the same effect.
U.S. Pat. No. 5,318,696 teaches a hydrocarbon conversion process based on a catalyst that consists of a macroporous molecular sieve and a molecular sieve having MFI structure with a silica-alumina ratio less than 30. This process is to produce gasoline of high octane number and increase the production of lower olefins, especially propylene, by modifying the catalytic cracking process.
U.S. Pat. No. 5,997,728 discloses a method for using a great amount of shape-selective cracking additive in the catalytic cracking process of heavy feed stocks. Said additive consists of 12-40% of ZSM-5 molecular sieve in an amorphous matrix, and has at least 10% of the inventory in the system so that the proportion of ZSM-5 in the catalyst exceeds 3%. This method can result in increasing the yield of lower olefins to a great extent while no further increase in aromatic output or loss in gasoline yield occurs.
CN 1034223A discloses a cracking catalyst for producing lower olefins, which consists of 0˜70% of clay, 5˜99% of inorganic oxides and 1˜50% of zeolite (based on weight of the catalyst), wherein the zeolite is a mixture consisting of 0˜25 wt % of REY or silica-rich γ-zeolite and 75˜100 wt % of phosphorus- and rare earth-containing silica-rich zeolite having five-membered ring. The catalyst has higher hydrothermal activity stability, conversion rate and C2=˜C4= yield than the catalyst containing HZSM-5 zeolite as an active component.
CN 1147420A discloses a phosphorus- and rare earth-containing molecular sieve having a MFI structure with the following formula in an anhydrous form regarding its chemical composition: aRE2O3bNa2OAl2O3cP2O5dSiO2, wherein a=0.01˜0.25, b=0.005˜0.02, c=0.2˜1.0, d=35˜120. When used for converting hydrocarbons at an elevated temperature, the molecular sieve has excellent hydrothermal stability of activity and better selectivity for lower olefins.
After ZSM-5 molecular sieve is modified with a phosphorus-containing compound, the stability of cracking activity can be improved, and the amount of the molecular sieve used can be decreased.
In U.S. Pat. No. 5,110,776, a method is disclosed for preparing a phosphorus-modified ZSM-5 molecular sieve catalyst. Said phosphorus-modifying process is carried out by dispersing the molecular sieve in an aqueous solution containing a phosphorous compound at pH value of 2˜6, The aqueous mixture containing phosphate modified zeolite is then combined with matrix precursors to form a slurry. The slurry is preferably spray dried to form the catalyst. The catalyst obtained results in increasing gasoline octane number without increasing the yields of dry gas and coke.
U.S. Pat. No. 5,171,921 discloses a phosphorus-modified ZSM-5 molecular sieve. After impregnated with a phosphorus-containing compound and then treated with water vapor at 500˜700° C., the molecular sieve having a silica-alumina ratio of 20˜60 has a higher activity when used in the reaction for converting C3˜C20 hydrocarbons into C2˜C5 olefins, as compared with the HZSM-5 that is not subjected to treatment with phosphorus compounds.
The methods for modifying a molecular sieve with metals and their uses have been reported as follows. For example, U.S. Pat. No. 5,236,880 discloses a catalyst comprising a MFI- or MEL-structured molecular sieve. When the catalyst with the modified molecular sieve added is used in conversion of paraffin, the octane number of C5˜C12 gasoline, the aromatic content, and/or the gasoline yield can be increased. Therefore, the catalyst that contains the molecular sieve thus modified can boost the octane number of gasoline and increase the yield of C3˜C4 olefins. The molecular sieve used is one modified with Group VIII metals, preferably Ni. The molecular sieve, after introduction of Ni, is subjected to thermal or hydrothermal treatment at a controlled severe temperature to make Group VIII metals and aluminum enriched on surface.
CN 1057408A discloses a cracking catalyst containing a silica-rich zeolite that has higher activity of catalytic cracking, wherein said silica-rich zeolite is a ZSM-5, βzeolite or mordenite which contains 0.01˜3.0 wt % of phosphorus, 0.01˜1.0 wt % of iron or 0.01˜10 wt % of aluminum, and is obtained by heating a H-type or K-type ZSM molecular sieve, aβ-zeolite or a mordenite having a silica-alumina ratio of higher than 15 at 350˜820° C., and then passing an aqueous solution of an aluminum halide, aqueous solution of a ferric halide or aqueous salt of an ammonium phosphate through it in a volume space velocity of 0.1˜10 hr−1.
The aromatization of lower alkane is an effective process for increasing utility value of lower alkane, and is one of efficient ways for optimizing the utility of carbon resources, because aromatization products can increase aromatics content of gasoline and thus the gasoline octane number
Researches on aromatization reactions of lower alkane have been done for many years to obtain a mixture of benzene, toluene, ethylbenzene and xylene (BTEX). Using alumino-silicates, especially molecular sieves having a high silica-alumina ratio as a catalyst for aromatization processes has been extensively researched, and especially extensive works have been done on using ZSM-5, ZSM-11, and/or ZSM-21 molecular sieves as the catalyst for the processes. In 1973, a process has been disclosed for aromatizing lower hydrocarbons (saturated and non-saturated) from cracking gasoline and coking gasoline or pyrolytic gasoline using a zeolite having a MFI structure (U.S. Pat. No. 3,756,942 and U.S. Pat. No. 3,845,150).
U.S. Pat. No. 4,288,645 discloses a process for producing aromatics mixture and hydrogen gas from light hydrocarbons containing at least 50% of propane using an alumino-silicate catalyst carrying Zn. This process requires that the light hydrocarbon contain desirably more than 60% of propane and less than 20% of methane and ethane. U.S. Pat. No. 4,175,057 and U.S. Pat. No. 4,180,689 disclose an aromatization reaction of propane and butane in the presence of a catalyst based on gallium and a MFI zeolite. After the patents, lots of other modified processes have been developed comprising modification of the catalyst (U.S. Pat. No. 4,795,844), increasing productivity (EP 252705, EP 050021 and U.S. Pat. No. 4,350,835) and modification of the system by introducing gallium (EP 120018 and EP 184927). Especially, EP 252705 discloses a zeolite-containing catalyst with a constraint index of 1-12, preferably the catalyst having a very high ratio of silica/alumina and containing 0.5-10% of gallium, and other elements that belong to Group VIII metals may also exist.
Like lots of organic catalytic reactions, there exists a problem for the catalyst used in aromatization reactions of decrease in catalyst activity resulting from carbon deposition on the catalyst surface, which requires frequent regeneration of the catalyst and thus causes many difficulties in industrial fixed-bed operations. Therefore, increasing carbon deposition resistance of the catalyst, improving catalyst stability and thus elongating reaction cycle and catalyst lifetime are the key to whether it can be industrially employed.
CN 1081938A discloses a process for modifying the aromatization catalyst by activating, wherein Zn/HZSM-5 type catalyst is activated with water vapor at an elevated temperature, and when the catalyst is used in the aomatization of pyrolytic gasoline, the catalyst lifetime is improved. However, the best result of lifetime obtained is only 29 hrs in a single pass. There is no mention of use in even lower hydrocarbons in the patent. CN 86 108104A and U.S. Pat. No. 4,636,483 of US Global Corporation disclose a composite catalyst that can be used for production of C2-C5 hydrocarbons with good anti-coking effect and an extended catalyst lifetime, wherein a main way is to improve the preparation of carrier by incorporating phorsphor-containing alimina into the crystalline silicate zeolite, dispersing the resultant material into microspheres in an elevated temperature oil, but the preparation process is complicated. CN 106200A discloses HZSM-5 and HZSM-11 zeolite catalysts modified with Zn—Pt and Ga—Pt respectively for aromatizing light hydrocarbons. But they are limited only to use with light hydrocarbon fractions having higher carbon number (C4-C9) from oil fields, and noble metal Pt is used and results in higher cost. U.S. Pat. No. 4,180,689 and U.S. Pat. No. 4,334,114 disclose Ga/HZSM-5 catalyst that is used in aromatization of C3-C12 hydrocarbons, aiming at modifying activity of the catalyst and selectivity of aromatics, but no problems on the carbon deposition resistance and stability of catalyst are involved. U.S. Pat. No. 4,157,293 patent describes in detail a HZSM-5 molecular sieve catalyst carrying zinc and incorporating a given amount of other elements, and the problem on decreasing activity of the aromatization catalyst due to loss of zinc during the reaction has been solved. It is pointed out in the document that a metal to be introduced is a metal of Group IB and Group VIII, and that germanium, rhenium, or a rare earth metal may also be added.
In the patents (U.S. Pat. No. 4,407,728 and EP 215579, EP 216491, EP 224162 and EP 228267), it has been found that the selectivity for aromatic compounds can be improved by adding platinum and palladium to the Ga and MFI-zeolite based catalyst, and carbon deposition can be inhibited on catalyst surface. However, the existence of these metals will enhance the formation of methane and ethane due to cracking. Later, it was discovered in research that the selectivity for aromatic compound can be further increased by introducing rhenium in the presence of platinum or palladium, but the quantity of C1-C2 lower alkanes in the product is also increased (U.S. Pat. No. 4,766,265). When the catalytic composition that contains copper or chromium and a MFI zeolite is used, fewer amount of methane can be formed, but the selectivity for aromatic compounds is less than that obtained by using a catalytic composition that contains gallium and a MFI zeolite (see P. Meriaudeau Zeolites: Facts, Figures, Future, 1423-1429, 1989; E. S. Shapiro, International Symposium on Zeolites as Catalysts, Sorbents and Detergent Builders RFA, 73, 1988). EP 474536 discloses a catalyst that contains a MFI zeolite, a platinum series of noble metal, a metal selected from the group consisting of Sn, Ge, In and Pb, an alkali metal and/or an alkali-earth metal component, which catalyst system improves the selectivity of aromatic compounds.
An elevated temperature required for the aromatization reaction of olefins and lower alkanes results in very short lifetime of the catalyst, because a phenomenon of severe coking and blocking occurs in the inner pores of the catalyst, which are closely related to cracking or polycondensation of the compounds in this reaction environment. CN 1284405A discloses a catalytic composition that contains gallium, at least one element selected from the group consisting of lanthanide series and a zeolite selected from the group consisting of MFI, MEL or MFI/MEL type zeolites. This catalyst system can result in higher selectivity of the aromatization of C3-C6 lower hydrocarbons than other catalytic system, especially the catalyst based on gallium and a MFI zeolite, and exhibits better resistance of deactivation, and as a result, elongates considerably lifetime.
Up to now, no report has been found that the molecular sieve having a MFI structure is modified simultaneously with both phosphorus and two metal elements.